466 



Messrs. F. W. Keeble and E. W. Gamble. 



least) a diurnal form passes over to the nightly phase. Though light 

 often induces and induces with marvellous rapidity a recovery from 

 the nocturnal phase, it is often powerless to overcome the habit of the 

 animal. The periodicity is only slowly worn down in the course of 

 two or three days. These changes express a nervous rhythm; per- 

 haps a profound and rhythmic course of metabolic events. The 

 reddish phase antecedent to the full nocturnal tint probably explains 

 the statement made by M. Malard,* that in darkness Hippolyte varians 

 becomes red. 



Periodicity is manifested in the colour-change of Hippolyte which 

 have been deprived of both their eyes. The assumption of, and 

 recovery from, the nocturnal phase is still effected, but more slowly 

 and erratically than in normal specimens. 



VII. Range of Colour-change. 



Adult animals when placed with weed of a new colour (the 

 light-intensity being as far as possible unaltered) are, under the 

 conditions of the laboratory, only capable of very slow sympathetic 

 colour-changes. Thus green Hippolyte placed on brown weed con- 

 serve their greenness for a week or more, but in the end give way and 

 become brown. Their subsequent recovery when placed with green 

 weed is more rapid. AVe have repeated such experiments in the open 

 time after time, and have found that the prawns were either quite 

 refractory or responded in this slow manner. Yet these same speci- 

 mens undergo the changes preceding and culminating in the nocturnal 

 colour and the succeeding recovery to their diurnal tint, with the 

 utmost readiness. The fact that prawns, refractory to sympathetic 

 colour-change can and do undergo a rapid change of tint when the 

 light-intensity or the quality of the light is altered, is shown by such 

 records as the following- A specimen, one of a large catch, incidentally 

 observed to be the blackest we have ever seen, became in a few 

 minutes transparent when put in a white porcelain dish. Further, a 

 ready and almost infallible means of producing transparent green 

 Hippolyte and even a colour hard to distinguish from the nocturnal 

 tint, consists in placing freshly caught prawns in a white porcelain dish, 

 and covering the top with a piece of muslin. Under these circum- 

 stances the change often takes place very rapidly (thirty seconds 

 to one minute). 



It is therefore necessary to distinguish at least three kinds of colour- 

 change in Hippolyte varians. First, the passage from the diurnal to the 

 nocturnal colour-phase followed by recovery to the colour of the pre- 

 vious day. In this case the phases form a rhythmic daily cycle. 

 Second, the colour-changes produced by artificially altering the light- 



* ' Bulletin cle la Societe Philomathique de Paris,' ser. 8, vol. 4, 1892, p. 28. 



